Listened to an interesting lecture this evening by Prof. Eric R. Varner regarding the reuse and repurposing of imperial images. He documented many examples of imperial portraits that upon closer inspection appeared to have been re-carved from the portraits of earlier emperors. The final example of his slideshow was the famous Colossus of Constantine:

Thanks, Voz - that's interesting and wasn't something I'd heard of before. The backwards slantiing forehead (where it had to be cut back to create some hair for Constantine), and well as the differing types of marble do seem quite convincing.

I wonder when reuse of earlier material for arches, statues, etc became the norm? Is this a sign of the declining resources of the late empire, or just a result of Roman practicality which had always been standard practice?

Just in case anyone else wants to save the text, I did a screen grab and OCR'd it (I manually checked/fixed it, so it should be accurate).

The coiffure has been reconfigured over the forehead and the forehead itself substantially carved back as a result. The eyes have been slightly recut, accounting for assymetricalities including the fact that the left eye is larger and has a larger fold of flesh beneath it than the right eye. Maxentius's aquiline nose has been modified into the hooked nose of Constantine. As a result, the bulge in the forehead over the brows is exaggerated and the nose appears too short and protrudes unnaturally in profile. Although the mouth has been recarved, causing it to be noticeably longer on the left side, it still retains its general Maxentian shape and receding lower lip. The recutting of the mouth has also caused the space between the lips to be much wider on the right side. The chin is fairly deeply cleft, a standard feature of Maxentius's portraits, but not usually on Constantine's. Removal of the cleft was clearly impractical as it would have substantially reduced the volume of marble in the lower section of the face. Constantine's portraits without cleft chins include: Copenhagen, 744a, inv. 3147 [F.,Johansen [1995b1 170, no. 74, with figs.); Madrid, Prado, 125 E (S.F. Schroder [1993]); New York. Metropolitan Museum of Art. inv. 26229 (H.P. L'Orange, [19841 69, 123. 48dr); Tunis, Musee du Bardo, C77 (H.P. L'Orange [1984] 53-56, 127. pl. 39 c-d). The reconfiguration has also caused the neck to be disproportionately large in comparison with the lower section of the face. Additionally, the top of the head seems too small in proportion to the overall dimensions of the face, but this would have been hidden by the addition of a metal diadem or crown; cuttings for such a headpiece are visible on both temples and on the left side of the coiffure.
F. Coarelli, citing unpublished observations by P. Zanker, mentions that the portrait in all probability is a reworked Maxentius ([1986] 2 and n.1), as also suggested earlier by H Jucker ([198b] 55-57), while C. Evers' has suggested that the head might instead be reworked from an image of Hadrian based on the distinctive treatment of the ears. The overwhelming physical details of the portrait, however, confirm that the image's most recent incarnation prior to its recutting to Constantine was as a likeness of Maxentius. Isotopic analysis of the marble seems to support a double reconfiguration, as most of the fragments (head, left foot, right foot, sections of the left leg, right calf, right knee, and the possible second right hand) are Parian marble, while the two sections of the neck, and right hand are Luna. Since it is unlikely that Maxentius would have had access to such unusually large blocks of Parian marble from which to create the portrait ex novo, it may have been recycled from a pre-existing representation of Hadrian. The putative Hadrianic colossus may have been damaged, thus necessitating the new right hand and repairs to the neck in Luna, and accessible in a marble depository (P. Pensabene, L. Lazzarini, and B. Turi [2002] 254). Maxentius's reuse of a Hadrianic original would accord well with his interest in that emperor as manifested in architectural projects like the rebuilding of the Temple Of Venus and Roma.

That is interesting and not surprising. The Arch of Constantine was erected in the same manner with other features "recycled" from other monuments. These were fast paced times and everything probably needed to be done yesterday!